Ultra high strength steel



United States Patent 3,148,980 ULTRA HIGH STRENGTH STEEL Peter Payson, Pittsburgh, Pa., assignor to Crucible Steel Company of America, Pittsburgh, Pa., a corporation of New Jersey No Drawing. Filed Oct. 24, 1960, Ser. No. 64,250 6 Claims. (Cl. 75-426) This invention relates to steels for constructional applications and, more particularly, to such steels exhibiting ultra high strength when subjected to conventional heat treatment. The invention also relates to low-alloy tool steels which may be conventionally heat treated to provide ultra high strength characteristics with high toughness and good ductility and to articles made therefrom.

So far as is known, there are no steels now available which as conventionally heat treated develop yield strengths of better than about 260,000 p.s.i. Admittedly many steels may be treated to hardness values over about Rockwell C60. If the well known correlation between hardness and yield strength is extrapolated from about Rockwell C45, it would be expected that steel heat treated to Rockwell 060 should have yield strength Well over 260,000 p.s.i. But actually steels now available either because of inherent brittleness or because of unfavorable residual stresses cannot support applied stresses over about 260,000 p.s.i. without fracture, or without sustaining a permanent set of more than 0.2%.

A practice recently developed designated ausforming does produce steel with yield strength well over 260,000 p.s.i. but this process involves appreciable section reduction by working the steel while it is at relatively low temperature but still in the austenitic condition, and then quenching it to form martensite. Because of the extreme difiiculty in forming or machining parts at such high strength levels, the size and shape as well as the constructional applications thereof are very limited. The ausforming process obviously is limited to relatively simple shapes and generally small sections such as sheet, strip and wire.

In addition to the property of high yield strength, which is all-important insofar as constructional application is concerned, many tool applications require a steel having high hardness combined with high toughness. High hardness is required to lengthen the useful life of a punch or die, and high toughness, as measured by impact strength, is needed to withstand shock loads. Examples of applications in which these two properties are essential are punches, forming tools, chisels, and shear blades. 7

At present, one of the best steels produced for these applications has a maximum hardness of about Rockwell CS6 with a standard V-notch Charpy impact strength of about 12 ft.-lbs. In general, while these property values are considered very good, for certain applications even greater hardness and toughness are required. While it is known to increase carbon content for the purpose of increasing hardness, the attendant disadvantage of a corresponding decrease in impact strength renders such knowledge nugatory and gives rise to a hiatus in the art. This hiatus in the art presents a problem of no small magnitude, particularly in the aircraft industry and in the field of rocketry and missiles, where criticality of ultimate tensile and yield strength are complexing factors which deserve no small consideration in the development of a steel designed to meet the increasingly high standards and requirements of the modern era.

Accordingly, a principal object of the present invention is to provide an ultra high strength steel for constructional applications.

Another object of the invention is to provide a steel which may be conventionally heat treated to exhibit ultra high strength and high hardness.

A further object of the invention is to provide a steel for shock resistant tools which may be conventionally heat treated to exhibit high impact strength, good ductility, and high hardness.

Still another object of the invention is to provide a low alloy steel which exhibits high tensile and yield strength, good ductility and toughness and high hardness upon being subjected to conventional heat treatment.

Yet another object of the invention is to provide improved articles of the aforesaid steels.

Other objects of the invention will be obvious from the following description.

The steels of the present invention, like conventional steels, can be annealed, formed or machined to approximately final dimensions while in the relatively soft annealed condition, and then heat treated by a quench and temper to develop ultra high strength. Broadly, the inventive steels include compositions within the following ranges:

Percent Carbon 0.50 to 0.65 Manganese Up to 0.75 Silicon 1.25 to 2.50 Nickel Up to 0.75 Chromium 1.00 to 2.00 Vanadium 0.10 to 0.50 Molybdenum 0.25 to 1.00 Iron- Balance In general, the inventive steels may include minor amounts of commercial impurities, including up to 0.025 each of phosphorus and sulfur, up to 0.20% copper and up to 0.05% aluminum.

Prefered steel compositions of the invention are given in the more narrow ranges listed below:

Percent Carbon 0.52 to 0.60 Manganese 0.05 to 0.50 Silicon 1.50 to 2.25 Nickel Up to 0.50 Chromium 1.10 to 1.80 Vanadium 0.15 to 0.40 Molybdenum 0.30 to 0.90 Iron- Balance In the preferred aspects of the invention, it is desirable to limit phosphorus and sulfur to a maximum of 0.020% each.

Optimum compositional ranges of the steels of the invention are as follows:

The optimum steels of the invention include no more than 0.010% each of phosphorus and sulfur.

During the course of the instant investigation it was assumed that to increase the strength of heat treated steel it was necessary to increase its carbon content. It was suspected that the desired ductility and toughness for very high hardness steel would require special melting to minimize nonrnetallics in the steel.

Accordingly, seven 30 pound heats of a modified standard deep hardening alloy tool steel designed to have excellent toughness properties at relatively high hardness levels were vacuum melted, made into ingots and forged into /s inch square bars. The chemical analyses are given in Table I.

TABLE I Chemical Analyses of Vacuum-Melted Steels Steel Mn P S St Ni Or v M0 57-153 0. 40 0. 74 0. 003 0. 004 2. 03 0. 05 1. 51 0. 23 0. 51 57-59. 0. 50 0. 41 0. 003 0.003 2.00 0.06 1. 90 0. 23 0. 40 57-100- 0.50 0. 40 0. 003 0. 003 1. 99 0.14 1. 40 0. 0. 32 57-10 0. 55 0.29 0.002 0. 004 2. 00 0.10 1. 73 0. 23 0. 40 57-104- 0. 70 0.03 0. 003 0 003 2. 01 0. 03 0. 90 0. 21 0. 52 57-165. 0. 73 0. 32 0. 003 0 002 2. 00 0. 07 1. 53 0. 32 0. 54 57-166 0. 70 0. 33 0. 002 0. 004 2. 00 0.14 1. 00 0. 0. s3 Air-Melted Standar 0. 45 1. 35 2 30 1.40 0.30 0.

10 impact specimens were tested on a Riehle impact testing machine using a pound hammer and a 4 foot drop.

Tensile and impact results are presented in Tables II and III, respectively.

It will be observed from Table II that Steel 57-158, having a carbon content of only 0.46%, has lower tensile szrength and yield strength values than the standard steel. Interpolation of the data for Steel 57-158 for a tempering temperature of 550 F. gives tensile strength values of about 305,000 p.s.i. and 312,000 p.s.i. and yield strength values of about 251,000 psi. and 254,000 p.s.i. in contrast to respective values of 3l4,000 p.s.i. and 260,000 p.s.i. for the standard steel. All of the other steels of T able H are superior to the standard steel in both tensile strength and yield strength.

TABLE II Tensile Test Data for Vacuum-Melted Steels Aus- Tern 0.2% El0nga Redn tenit paring Hard- Tensile Onset tionin tionin Steel izing Temp. ness Strength Yield one Area Temp. C F.) (Re) tpsi.) Strength inch (per- F.) (p.s.1.) (percent) cent) 57-15s.400 1, 050 350 53 343,000 204,000 7.1 12.4 400 57 322, 000 223, 000 12. 4 33. 0 500 57 310, 000 251, 000 13. 3 32. 7 000 57 303, 000 257, 000 14. 2 21. 0 400 53 337, 000 240, 000 12. 0 32. 7 500 57 314, 000 203, 000 10. 0 a1. 3 000 50 315, 000 203, 000 12. 0 3g, 3

1165" 350 50 350,000 203,000 7.1 13.0 400 53 330, 000 233, 000 12. 4 25. 9 500 50 330, 000 200, 000 s. 0 25. 0 000 57 317, 000 205,000 10. 0 35. 1 400 53 333 000 242,000 12. 0 35, 0 500 57 321, 000 274, 000 10. 0 34. 5 50C 1 650 000 57 321, 000 273,000 10.0 37.1 a

400 50 343, 000 248, 000 10. 0 23. s 500 57 337,000 273,000 9. 0 20. 3 r2 5 2 3s 2132 32 0 52, 5 20.2 57-102550 1,550{ 500 59 333,000 281,000 8.0 23.0 2 2523 's 35 1 9, 0 5. 4.0 400 30 300, 000 243, 000 0. 0 5. 2 57 162-550 500 53 341,000 270,000 7.0 21.0 2 g 58 000 000 2. 24. 4 00 7,000 .000 10.0 57-104700 1,550{ 500 59 340,000 235,000 0.0 13.3 000 29 342, 28g, 4. 0 10. s 350 1 3-1, 10 1.3 0.3 400 01 342,000 224,000 1.8 1. 0 57 164-700 117ml 500 53 347,000 285,000 4.4 4.3 $83 2? 35337 18 350328 3 .0 0.2 57-105730 1,000{ 500 00 343,000 277,000 4.0 15.2 323 23 337 7333 05888 53 0.0 400 01 352 000 212,000 2. 0 1.4 57 117ml 500 50 3521000 273,000 8.0 10.3 23 1 00 .0 4.0 57-100.?00 1, 550{ 500 59 3101000 2611000 1.8 4.0 20 53 348, 888 298, 000 4. 0 s. 5 5 02 317, 00,000 1.3 0. 7 a 400 31 337,000 232000 0.0 0.2 57 1350i 20 23 321,3 0 22113 0.0 11.2

0 3 5 2 2 0.0 s. Air-Melted Standard.-." 1,700 550 50 31 5000 2001000 0.0 30.0

1 In 2-incl1 gauge length.

N01E.Steels were austenitized for 1 hour, oil quenched, and tempered for 2 hours at lndtcated temperatures.

ness (Re) V-notch Hard-- Impact Strength (p cent) 1 inch Area (P cent) TABLE V Steels 0.2% Elonga- Reduc- Oharpy Onset; tion in tion in Yield Strength Tempered at 400 Tempered at 500 F.

Tempered at 600 F.

Ultimate Tensile Strength (p.S.i.)

Steel Mechanical Test Results of Vacuum Induction Melted Tempering Temperature TABLE III Steels (ft-lb.)

ness Strength ness Strength ness Stren O6 5421 6 4 um EH21 1111613571 0 m .afleumoaomiaiiaemai F mun 6 d 666777838869996 T 555555555555555 3 R nl\ h 2 6 565 1 2BU1H111661749M a 6 77779880070006 M m Rowe-0555555556665 H 66506 3 MW BB11111B7676591 3 l 1 v r 1 1 1 1 i y y i i 1 v 1 F m fi 21111111 1 4 d 778888080101017 M 555555656666665 t C a m T.

Charpy V-Notch Impact Test Data for Vacuum-Melted Steel Air-melted Standard NOTE.-St80l5 were austenitized for 1 hour, oil quenched, and tempered for 2 hours at indicated temperatures.

Table III indicates that Steels 57-164, 57-165 and 57- 166, having carbon contents of 0.70% or higher, have exhibits the same hardness as the pering temperatures of 500 F. and 600 F. although it has superior impact strength. All the other steels of Table Ill, i.e., Steels 57-159, 57-160 In order to more substantially validate the inventive compositions additional work was undertaken, particu- Accordingly, four 50 pound vacuum induction melted heats were made into ingots, forged to bars 1% inches The chemical analyses are given in Table IV.

average impact strength values which are inferior to those of the standard steel. Steel 57-158, having a carbon content of 0.46%,

standard steel for tem and 57-162, having carbon contents in the range 0.50- 0.65%, exhibit average impact strength and hardness values superior to the standard steel for tempering temperatures ranging from 400 F. to 600 F.

larly with the view in mind of studying the influence of nickel and modifications of the molybdenum and vanadium contents.

square in cross section and spheroidize-annealed.

TABLE IV 1 Tested at room temperature.

Table V (note Steels 58-316 and 58-284) indicates that tensile and yield strength as well as hardness decrease upon the addition of nickel. It will be noted, however,

Steel 58-317, which contains a higher molybdenum 5 content than that of Steel 58-316, the base heat, exhibits On the other hand, Steel 58-288, which contains both a higher vanadium content and a higher molybdenum content, exhibits somewhat lower strength and toughness as well as somewhat lower hardness and ductility.

In connection with the elfect of the compositional elements upon the physical properties of the subject steels, it was felt that the influence of manganese deserved some attention. Accordingly, an investigation along this line was undertaken. Furthermore, the steels studied were air 2% that toughness is somewhat improved thereby. 82 so little or no change in properties.

VMo

Si Ni melted in an effort to establish the influence of air melting on the basis of a larger number of heats.

As in the original investigation 30 pound induction heats were made into ingots and forged to bars /3 inch square, with the exception that the heats were air-melted rather than vacuum-melted. The complete analyses are given in Table VI.

Mechanical Test Data for Air-Melted Steels [Prior condition: Annealed (austenitlzed at 1700 F. for

1 hr., quenched in Warm oil, and tempered at 500 F. for 2 hrs.)]

Ultimate 0.2% Elonga- Reduc- Charpy Tensile Offset tion in tion in V-notch Hard- Stcel Strength Yield 1 inch Area Impact; ness (p.s.i.) Strength (per- (per- Strength (3.)

(p.s.1 cent) cent) (IL-lb.)

314, 000 200, 000 2 e 11 58 58471 l 320,000 200,000 27 14 58 318, 205, 000 12 35 5s 58178 320,000 200,000 10 as 15 5s 31s, 250, 000 14 31 15 5s 584/2 319,000 202,000 12 31 10 5s 315, 000 259, 000 4 0 15 5s 314,000 251,000 14 30 14 51 Examination of Tables VI and VII indicates that manganese content is inversely proportional to yield strength, i.e., as the manganese content increases, the yield strength decreases. It is of particular importance to note that as the manganese content increases from 0.44% (Steel 58- 178) to 0.82% (Steel 58-172) the yield strength approaches or decreases below the value for the standard steel (see Table II). Steels 58-172 and 58-173 having manganese contents greater than 0.75%, fail to excel the standard steel in yield strength, and are therefore not within the purview of the invention concept. Steels 58-171 and 58-178, having manganese contents less than 0.75% are notably superior to the standard steel not only in yield strength but also in tensile strength, impact strength and hardness and are, therefore, exemplary of the steels of the invention.

Comparison of test results for both vacuum-melted and air-melted steels of the invention indicates that both tensile and yield strength values are superior in the case of vacuum-melted steels. This is clearly shown by Table VIII, which comprises comparative data taken from Tables II and VII.

TABLE VIII Comparison of Vacuum and Air-Melted Steels Tensile 0.2% Ofi- Impact Hard- Steel Type of Melting Strength set Yield Strength uess (p.s.i.) Strength (ft. lb.) (Ru) 57-102--- Vacuum Indnctiom. 330,000 281,000 13,115 58 58-171." Air Induction gg ggg 11,14. 58

NOTEr-StGQlS were austenitized at 1700 F. for 1 hour, quenched in warm oil, and tempered at 500 F. for 2 hours.

Although vacuum induction melting constitutes a preferred mode of making the steels of the present invention,

it is to be understood that any melting process, including air melting, is applicable. Conventional electric arc melting may be employed with or without the practice of special deoxidation or degassing. Further, vacuum arc remelting or any modification of the induction melting process may be employed.

Having thus described the invention so that others skilled in the art may be able to understand and practice the same, I state that what I desire to secure by Letters Patent is defined in what is claimed.

I claim as my invention:

1. An ultra high strength steel consisting essentially of 0.52 to 0.60% carbon, 0.05 to 0.50% manganese, 1.50 to 2.25% silicon, up to 0.50% nickel, 1.10 to 1.80% chromium, 0.15 to 0.40% vanadium, 0.30 to 0.90% molybdenum, the balance substantially iron.

2. An ultra high strength steel consisting essentially of 0.53 to 0.58% carbon, 0.10 to 0.20% manganese, 1.70 to 2.10% silicon, up to 0.20% nickel, 1.10 to 1.40% chromium, 0.20 to 0.35% vanadium, 0.40 to 0.70% molybdenum, the balance substantially iron.

3. A steel according to claim 1, said steel being characterized by an 0.2% offset yield strength of at least 275,000 p.s.i. and a hardness of greater than Rockwell C58 upon being subjected to a conventional heat treatment comprising austenitizing, quenching and tempering.

4. A steel according to claim 2, said steel being characterized by an 0.2% offset yield strength of at least 275,000 p.s.i., a hardness of greater than Rockwell C58 and a V-notch impact strength of at least 10 ft.-lbs. at room temperature upon being subjected to a conventional heat treatment comprising austenitizing, quenching and tempering.

5. A shock resistant tool comprising a steel according to claim 1, said steel being characterized by being conventionally heat treatable to provide properties of high impact strength, good ductility and high hardness.

6. A shock resistant tool comprising a steel according to claim 2, said steel being characterized by being conventionally heat treatable to provide properties of high impact strength, good ductility and high hardness.

References Cited in the file of this patent UNITED STATES PATENTS 2,645,574 Clark July 14, 1953 

1. AN ULTRA HIGH STRENGTH STEEL CONSISTING ESSENTIALLY OF 0.52 TO 0.60% CARBON, 0.05 TO 0.50% MANGANESE, 1.50 TO 2.25% SILICON, UP TO 0.50% NICKEL, 1.10 TO 1.80% CHROMIUM, 0.15 TO 0.40% VANADIUM, 0.30 TO 0.90% MOLYBDENUM, THE BALANCE SUBSTANTIALLY IRON. 